Periodically, I've looked at possibly adding these kind of extensions. 

The argument in favor in that it is easy to do and is an obvious extension with plausible utility.  

The main arguments against is that it represents feature creep far removed from the original intended use cases (the tool is primary about counting and trys not to venture into elementwise arithmetic on sparse vectors.)   

It is tempting to add a new feature that might sometimes be useful, but we should also worry that usability and learnability are impaired if the class becomes less cohesive, less thematic, and less focused on unified design goals.

The other factor against adding the feature is that since the Counter is just a dict subclass, it is easy for users to just manipulate the data directly.  We're not really adding much that a person can't already easily do themselves.